The present invention relates to a method for controlling the ignition timing of an internal combustion engine such as an automotive engine and more particularly to a method of controlling the ignition timing at a time when a maximum limit ignition timing for obtaining maximum torque without causing knocking is largely retarded in accordance with an octane number of gasoline used in the engine or engine operating conditions.
A learning control system for correcting the ignition timing has been proposed. The control system is adapted to advance the ignition timing so as to produce a maximum torque as long as the level of the engine knock does not exceed a tolerable level. If the knock occurs, the control system operates to retard the ignition timing by a predetermined value. A maximum limit ignition timing for producing the maximum torque without causing knocking depends on a type of the engine and on the octane number of the fuel used in the engine. Namely, when high-octane gasoline or mixture thereof with regular gasoline is used, the ignition timing is advanced compared with that for low-octane gasoline to increase the torque. If the octane number varies, or if a large change in engine operating conditions occurs, the ignition timing varies accordingly so that it is necessary to correct the timing through a learning control system.
The maximum ignition timing is advanced as the octane number of the fuel increases. However, it is not advisable to advance the timing in accordance with the octane number over a limit ignition timing for producing a maximum torque dependent on the engine operating conditions because the torque is reduced.
Japanese Patent Application Laid-Open 61-157771 (U.S. Pat. No. 4,694,801) discloses a control system wherein the operation thereof is divided into an entire correction and a individual correction. In the entire correction, all of learning correcting quantities stored in a memory are roughly corrected to set the quantities to values approximate to limit values. In the individual correction, the learning correcting quantities are individually and finely corrected, in accordance with engine speed and intake air pressure. Thus, when the value of the fine correcting quantity becomes large and exceeds a predetermined value, the correcting operation is returned to the rough correction. At that time, the value is the deviation of the actual ignition timing from a desired value.
However, it takes a long time for a calculation of a correct ignition timing because an error between a desired ignition timing and an actual ignition timing is large. Accordingly, the calculation changes from the fine correction to the rough correction, so that a delay may occur. The delay of the returning is a problem in the case that the ignition timing is advanced because the knocking is not likely to occur.
To the contrary, when the high-octane gasoline is changed to the low-octane gasoline, the limit ignition timing is largely retarded so that the delay in returning to the rough correction causes a frequent knocking. Thus, it is necessary to quickly change the fine correction to the rough correction.
Moreover, the knocking induces more knockings because the knocking causes the temperature in the cylinder to rise.